The healthy vaginal microbiota is typically characterized by the predominance of Lactobacillus species (including L. crispatus, L. jensenii, L. gasseri, and L. iners). These bacteria are thought to protect against pathogens through a variety of mechanisms, including competitive inhibition, secretion of bacteriocins (substances that inhibit the growth of bacteria), and the production of lactic acid, which lowers the vaginal pH and has immunomodulatory effects (1). Consequently, vaginal dysbiosis is currently defined by the presence of polymicrobial bacterial populations with reduced or absent lactobacilli. This low-lactobacillus state characterizes bacterial vaginosis, a clinical condition that is a common cause of vaginal symptoms in reproductive-aged women (2). Decreased bacterial diversity in the gut has been linked to a number of pathologic conditions (3), whereas increased vaginal bacterial diversity has been associated with a variety of poor patient outcomes including preterm birth and the acquisition and spread of sexually transmitted infections (4, 5). On page 938 of this issue, Klatt et al. (6) add to this list a decrease in the efficacy of the vaginal antiretroviral gel tenofovir to prevent HIV acquisition.